It is known, for example, from WO 95/01772 that cationic dyes can be used to dye organic material, for example keratin, silk, cellulose or cellulose derivatives, and also synthetic fibers, for example polyamides. Cationic dyes exhibit very brilliant shades. A disadvantage is their unsatisfactory fastness to washing.
R. S. Asquith, P. Carthew and T. T. Francis describe in JSDC from May 1973, pages 168-172 that ortho-azo disulfide dyes do not lead to covalent bonding with keartin fiber of wool, and that para-azo disulfide dyes underwent only at high concentration some covalent bindings with wool.